One dimensional (1D) nanostructures have received tremendous attention in the field of electronics and optoelectronics since the discovery of carbon nanotubes, as reported by S. Iijima, Nature 1991, 354, 56. In particular, 1D semiconductor nanostructures are considered to be critical building blocks for nanoscale electronic and optoelectronic devices. To improve performance of these nanodevices, it is important that the efficiency of 1D semiconductor nanostructures be increased. Since surface defects are prominent due to large surface to volume ratio, efficiency of nanostructures and hence their performance in nanodevices could be improved by reducing surface defects.
Thus the focus now is on developing synthesis strategies for effective surface passivation of 1D nanostructure that minimizes surface defects. To date, the best surface passivation approach is the creation of radial hetero-structures such as core/shell nanostructures. In a typical surface passivation procedure, a shell structure of a wide band-gap material is created over the core. In an effectively surface passivated core/shell nanostructure, the core is completely covered by an epitaxial shell with minimal lattice mismatch between the core and the shell. Due to limited availability of appropriate shell layer material, to date, only a few 1D core/shell nanostructures have been reported in the literature including Ge/Si, as reported by L. J. Lauhon, et al. in Nature 2002, 420, 57; RuO2/TiO2 as disclosed by Y. L. Chueh, et al. in Advanced Materials 2007, 19, 143; CdSe/CdS as reported by D. V. Talapin, et al., in Nano Letters 2003, 3, 1677; CdSe/ZnS as reported by T. Mokari et al. in Chemistry of Materials 2003, 15, 3955; CdS/ZnS as disclosed by Y. J. Hsu, et al. in Chemical Communications 2004, 2102; and CdSe/ZnSe as disclosed by H. Lee et al. in Journal of Chemical Physics 2006, 125.
Traditionally, 1D core/shell nanostructures have been synthesized using high-temperature vapor deposition methods such as chemical vapor deposition (CVD) and metallo-organic chemical vapor deposition (MOCVD). These methods are sophisticated and demand controlled step-wise supply of core and shell elements in the vapor state. It is therefore desirable to develop a relatively simple method of synthesizing 1D core/shell nanostructures. Moreover it is desired that methods are developed to synthesize core/shell nanostructures with high aspect ratio, such as CSNWs. In this context, hot-phase chemical routes appear to be more attractive than traditional CVD methods. However, synthesis of 1D nanostructure with a large aspect ratio (i.e. length to diameter) using hot phase chemical route is challenging. So far, only short nanorods have been reported in the literature, for example, L. Manna, et al. in Journal of the American Chemical Society 2002, 124, 7136; however, there are inherent limitations of the method. Synthesis of dopant based thin (<10 nm) 1D core/shell nanostructures with high aspect ratio such as core shell nanowires (CSNWs) has not been reported using the aforementioned methods.
The cadmium sulfide (CdS) based nanostructures are among the most widely studied semiconductor nanomaterials due to their widespread applications in the field of electronics and optoelectronics. Both cadmium sulfide (CdS) and zinc sulfide (ZnS) are II-VI group semiconductors and they have similar crystalline structure. However, ZnS has higher band gap energy than CdS (˜3.7 eV versus 2.4 eV when measured in the bulk). Owing to relatively higher band gap energy, ZnS has been widely used as surface passivating shell material for CdS. The divalent Mn2+ ions are appropriate as dopants for both of these II-VI group semiconductor materials. Upon excitation, the Mn2+ doped semiconductors produce characteristic bright yellow luminescence (˜585 nm emission) due to the 4T1=>6A1 Mn2+ ion transition as reported by R. N. Bhargava et al. in Physical Review Letters 1994, 72,416 and S. Biswas et al. in Journal of Physical Chemistry B 2005, 109, 17526. Also, as building blocks in nanoscale optoelectronic devices as discussed by H. J. Choi, et al. in Advanced Materials 2005, 17, 1351 and D. S. Han et al. in Applied Physics Letters 2005, 86, these Mn2+ ions doped CdS semiconductors are utilized as dilute magnetic semiconductors.
To date, a few reports on the CdS:Mn/ZnS core/shell nanoparticles have been published but none of the published references discuss the synthesis of CdS:Mn/ZnS CSNWs. The publications that discuss CdS:Mn/ZnS are by H. Yang et al in Advanced Functional Materials 2004, 14, 152; S. Santa, et al. in Journal of the American Chemical Society 2005, 127, 1656; and H. S. Yang, et al. in Advanced Materials 2006, 18, 2890.
An easy, simple, inexpensive synthesis method for core shell nanowires is needed and would significantly benefit the semiconductor industry. The synthesis method and novel one dimensional (1D) core shell semiconductor nanowires of the present invention solve many problems and overcome many limitations in the prior art.